Optimal coordination of directional overcurrent relays in distribution systems with DGs and FCLs considering voltage sag energy index

Abstract

Parallel analysis of power quality and protection coordination has become an important part of modern power system studies. Nowadays, the coordination of directional overcurrent relays (DOCRs) is significantly affected by the emerging of distributed generations (DGs). Using voltage sag energy index, a new approach is proposed for optimization of DOCRs coordination in distribution systems in the presence of DGs, Fault current limiters (FCLs), and voltage-sensitive buses. This approach aims to minimize an objective function which comprises three key elements: operating time of relays, voltage sag energy index, and voltage sag duration. The impedance of FCL and the time setting multiplier, current setting and characteristic of DOCRs are the optimization variables. Voltage tolerance curve and voltage sag energy index are also used to show the improvement of voltage sag characteristics and prioritization of answers. Simulation results show that, the operating time of DOCRs and voltage sag characteristics are significantly improved.